Observation hole device for optical instruument

An eye cup moving mechanism of an optical device includes: an eyepiece lens holding barrel that houses and holds an eyepiece lens; an eyepiece outer casing that is capable of sliding against an external circumferential portion of the eyepiece lens holding barrel; and an eye cup cover provided at the eyepiece outer casing. The eyepiece lens holding barrel includes a first regulating member provided at the external circumferential portion thereof; and the eyepiece outer casing includes a second regulating member that comes in contact with the first regulating member to regulate a movement of the eye cup cover along an optical axis of the eyepiece lens.